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The traditional init process was originally only responsible for bringing the computer into a normal running state after power-on, or gracefully shutting down services prior to shutdown. As a result, the design is strictly synchronous, blocking future tasks until the current one has completed. Its tasks must also be defined in advance, as they are limited to this prep or cleanup function. This leaves it unable to handle various non-startup-tasks on a modern desktop computer elegantly, including:

The addition or removal of USB flash drives and other portable storage or network devices while the machine is running

The discovery and scanning of new storage devices, without locking the system, especially when a disk may not even power on until it is scanned

The loading of firmware for a device, which may need to occur after it is detected but before it is usable

Upstart's event-driven model allows it to respond to events asynchronously as they are generated.[2]

Upstart operates asynchronously; it handles starting of the tasks and services during boot and stopping them during shutdown, and also supervises the tasks and services while the system is running.

Easy transition and perfect backward compatibility with sysvinit were the explicit design goals;[3] accordingly, Upstart can run unmodified sysvinit scripts. In this way it differs from most other init replacements (beside systemd and OpenRC), which usually assume and require complete transition to run properly, and do not support a mixed environment of traditional and new startup methods.[4]

Upstart allows for extensions to its event model through the use of initctl to input custom, single events, or event bridges to integrate many or more-complicated events.[5] By default, Upstart includes bridges for socket, dbus, udev, file, and dconf events; additionally, more bridges are possible.[6]

Upstart was first included in Ubuntu in the 6.10 (Edgy Eft) release in late 2006, replacing sysvinit. Ubuntu 9.10 (Karmic Koala) introduced native Upstart bootup as of Alpha 6.[7] In turn, after the Debian project decided to adopt systemd on a future release in 2014, Mark Shuttleworth announced that Ubuntu would begin plans to migrate to systemd itself to maintain consistency with upstream.[8]

Linux distributions that support or have supported Upstart to some extent, but moved away since or no longer use it as their default init system:

Debian decided that systemd will be the default init system beginning with the jessie release,[10] after considering switching to Upstart.[11] It was eventually removed from the Debian archives in December 2015.[12]

Ubuntu finished the switch to systemd as its default init system in version 15.04 (Vivid Vervet), with the exception of Ubuntu Touch.[13]